1. Field of the Invention
The present invention relates to an endless belt assembly, a fixing device, and an image forming apparatus employing the same, and more particularly, to an endless belt assembly including an endless looped belt entrained around multiple rollers applicable to a fixing device that fixes a toner image in place on a recording medium with heat and pressure, and an electrophotographic image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of those imaging functions, employing such an endless belt assembly.
2. Description of the Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of those imaging functions, a toner image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and settling the toner particles with heat and pressure.
Various types of fixing devices are known in the art, most of which employ a pair of generally cylindrical looped belts or rollers, one being heated for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip through which a recording medium is passed to fix a toner image onto the recording medium under heat and pressure.
A specific type of such fixing device comprises a fuser belt assembly that includes an endless, looped fuser belt entrained around multiple support rollers disposed parallel to each other to define a path of movement in which the fuser belt rotates during operation. Due to low thermal capacity of the fuser belt resulting in a short warm-up time required upon activation, this type of fixing device is employed in modern printing systems that require high-speed, power-efficient fixing capabilities.
One problem associated with a multi-roller, fuser belt assembly depicted above is lateral misalignment or displacement of the fuser belt from its desired path of movement. The problem occurs where the fuser belt rotating around the multiple support rollers slips laterally in an axial direction parallel to longitudinal, rotational axes of the belt support rollers, and ultimately climbs sideways over the outer circumference of the support roller to a position deviating from the original path of movement around the multiple support rollers.
Lateral displacement of the fuser belt, if not corrected, would result in various operational failures, such as imaging defects and belt breakage, due to improper positioning of the fuser belt relative to the belt support roller. Such a problem is particularly pronounced in high-speed, power-efficient applications where the belt support roller is formed of silicone rubber for obtaining a reduced thermal capacity, or is dimensioned to have a smaller diameter for accommodating an increased circumferential speed.
To date, various techniques have been proposed to counteract lateral displacement of an endless looped belt.
For example, one such technique provides a belt tracking mechanism for an endless belt entrained around multiple support rollers, including a pair of ribs or protrusions each extending along a side edge on an interior circumferential surface of the endless belt adjoining the support rollers. During operation, the belt rib contacts an end face of the belt support roller so as to restrict lateral movement of the endless belt, which then can travel in a desired path of movement.
Another technique provides a belt tracking mechanism including an endless belt with a pair of circumferential edge ribs each facing an end face of a support roller, as well as a friction member mounted on each end face of the support roller to frictionally contact the belt rib to restrict lateral movement of the endless belt during rotation.
With reference to FIGS. 1A and 1B, which are fragmentary perspective and elevational views, respectively, of such a belt tracking mechanism, an endless belt 91 is shown entrained around a support roller 92 rotatable about a roller shaft 92a extending in a longitudinal, axial direction. A circumferential edge rib 91a extends along a side edge on an interior circumferential surface of the endless belt 91. An annular, friction flange 93 is supported on the roller shaft 92a to rotate freely, i.e., independent of and relative to the support roller 92 as it contacts a journal of the rotating support roller 92.
In such an arrangement, provision of the friction flange 93 reduces load on the endless belt 91 and the support roller 92 where lateral displacement of the endless belt 91 causes the edge rib 91a to interfere with the end face of the support roller 92 to destabilize rotation of the endless belt 91, which would otherwise aggravate the belt tendency to climb over the roller circumference upon minor slippage of the belt during rotation.
Still another technique provides an improved belt tracking mechanism similar to that depicted above, wherein the annular friction flange 93 of the support roller 92 and the circumferential edge rib 91a of the endless belt 91 have their interfacing surfaces beveled or inclined to effectively prevent the endless belt 91 from climbing over the outer circumference of the support roller 92.
Although advantageous for their intended purposes, the belt tracking techniques described above would not function properly, where the belt rib interferes with the end face of the belt support roller to cause undue load on the rotating belt and rollers, resulting in destabilized rotation of the endless belt, and therefore aggravated belt tendency to climb over the roller circumference.
As mentioned earlier, such malfunctioning may be alleviated by providing a friction member as that depicted above with reference to FIGS. 1A and 1B. Unfortunately, however, this is not the case with high speed applications where the belt support roller is relatively large in diameter. As shown in FIG. 1C, with such a large-diameter support roller 92 supporting the endless belt 91, the annular friction flange 93 tends to tilt relative to the longitudinal axis of the support roller 92 as it interferes with the edge rib 91a upon lateral displacement of the endless belt 91, resulting in a non-uniform, inconsistent contact between the annular friction flange 93 and the roller journal, which adversely affects rotation of the annular friction flange 93, thereby causing undue load on the rotating endless belt 91 and the support roller 92.
Recent trends in printing systems toward higher processing speed involve accelerating rotational speed of the multi-roller belt assembly with a corresponding increase in the diameter of the belt support roller, which makes it even more difficult to provide a reliable belt tracking mechanism with high immunity against belt failure due to lateral displacement of an endless looped belt.